The present invention is concerned with processes for making dehydration products from sugars and/or from sugar alcohols, and more particularly but without limitation, to acid-catalyzed processes for making such dehydration products as hydroxymethylfurfural (HMF), levulinic acid and furfural from the pentose and/or hexose sugars and for making isohexides, such as isosorbide, from hexitols such as sorbitol.
Those skilled in the art have long appreciated that agricultural raw materials provide an inexpensive and renewable source of carbohydrates that could in turn be made into a variety of useful materials which are now made or derived from non-renewable feedstocks or into other, biobased or renewable source-derived materials that may have similar properties and utilities. Certain of these other, biobased or renewable source-derived materials have been conceived or proposed (as further elaborated below) based upon the dehydration products that can be made from, e.g., the pentose and hexose sugars or sugar alcohols such as sorbital which may be obtained by hydrogenating dextrose.
For its part, the particular sugar dehydration product HMF and its related 2,5-disubstituted furanic derivatives are of interest for a variety of applications and uses. More particularly, due to its various functionalities, it has been proposed that HMF could be utilized to produce a wide range of products such as polymers, solvents, surfactants, pharmaceuticals, and plant protection agents, and HMF has been reported to have antibacterial and anticorrosive properties. HMF is also a key component, as either a starting material or intermediate, in the synthesis of a wide variety of compounds, such as furfuryl alcohols, aldehydes, esters, ethers, halides and carboxylic acids.
In addition, HMF has been considered as useful for the development of biofuels, fuels derived from biomass as a sustainable alternative to fossil fuels. HMF has additionally been evaluated as a treatment for sickle cell anemia. In short, HMF is an important chemical compound and a method of synthesis on a large scale to produce HMF absent significant amounts of impurities, side products and remaining starting material has been sought for nearly a century.
Unfortunately, while it has long been known that HMF could be prepared from readily obtainable hexose carbohydrates, for example by acid-catalyzed dehydration methods, a method which provides HMF economically, with good selectivity and in high yields, has yet to be found. Complications for selectivity and yield arise from the rehydration of HMF, which yields by-products, such as, levulinic and formic acids. Another unwanted side reaction includes the polymerization of HMF and/or fructose resulting in humin polymers, which are solid waste products. Further complications may arise as a result of solvent selection. Water is easy to dispose of and dissolves fructose, but unfortunately, low selectivity and increased formation of polymers and humin increases under aqueous conditions. The purification of HMF has also proved to be a troublesome operation. On long exposure to temperatures at which the desired product can be distilled, HMF and impurities associated with the synthetic mixture tend to form tarry degradation products. Because of this heat instability, a falling film vacuum still must be used. Even in such an apparatus, resinous solids form on the heating surface causing a stalling in the rotor and frequent shut down time making the operation inefficient. Prior work has been performed with distillation and the addition of a non-volatile solvent like PEG-600 to prevent the buildup of solid humin polymers (Cope, U.S. Pat. No. 2,917,520). Unfortunately, the use of polyglycols leads to the formation of HMF-PEG ethers.
As to another sugars dehydration product, namely, levulinic acid, the National Renewable Energy Laboratory (Denver, USA) has identified levulinic acid as one of a number of key sugar-derived platform chemicals that can be produced from biomass. Levulinic acid can be used to produce a variety of materials for a variety of uses, including succinic acid, 1,4-butanediol, 1,4-pentanediol, tetrahydrofuran, gamma valerolactone, ethyl levulinate and 2-methyl-tetrahydrofuran, for example, for producing resins, polymers, herbicides, pharmaceuticals and flavoring agents, solvents, plasticizers, antifreeze agents and biofuels/oxygenated fuel additives.
Rackemann and Doherty, “The Conversion of Lignocellulosics to Levulinic Acid”, Biofuels, Bioproducts & Biorefining, 5:198-214 (2011) provide an overview of current and potential technologies which had been publicly identified or suggested, for producing levulinic acid from lignocellulosics. The “most promising” commercial process according to the reviewers utilized the Biofine™ technology developed by Fitzpatrick (and described for example in U.S. Pat. No. 5,608,105), involving a two-stage acid-catalyzed process wherein in a first, plug flow reactor a carbohydrate-containing material (primary sludges from paper manufacture, waste paper, waste wood, agricultural residues such as corn husks, corn cobs, rice hulls, straw, bagasse, food processing wastes from corn, wheat oats and barley) is dehydrated to 2,5-hydroxymethylfurfural (HMF) at from 210 to 230 degrees Celsius for less than 30 seconds, and then levulinic acid is produced in a second reactor at 195 to 215 degrees Celsius for 15 to 30 minutes. The reviewers conclude that further improvements must be made, however, for the cost-effective production of levulinic acid from biomass, in particular citing yield losses from re-polymerization and side reactions.
The Rackemann and Doherty review (at page 203) further recognizes that levulinic acid may also be obtained from furfural, another sugars dehydration product—from pentoses in the hemicellulosic fraction of biomass—by catalytically reducing the furfural through the addition of hydrogen to form furfuryl alcohol, and then converting the furfuryl alcohol to levulinic acid and alkyl levulinates. Similarly, in U.S. Pat. No. 7,265,239 to Van De Graaf et al, furfuryl alcohol and water are converted to levulinic acid with the use of a porous strong acid ion-exchange resin, or furfuryl alcohol with an alkyl alcohol are converted to an alkyl levulinate. Still earlier references describe other means for converting the pentoses in the hemicellulosic fraction of biomass into levulinic acid and/or its derivatives, by means of furfural and furfuryl alcohol, see, for example, U.S. Pat. Nos. 2,738,367; 4,236,012; 5,175,358; 2,763,665; 3,203,964; and 3,752,849.
The dehydration products that can be made by the acid-catalyzed dehydration of sugar alcohols, in particular, hexitols such as sorbitol, have also been the subject of extensive work. Isosorbide, also known as 1,4,3,6-dianhydrosorbitol, is now commercially produced and marketed as a monomer for imparting renewable content to polyesters and polycarbonates, and has been used as a pharmaceutical intermediate.
A variety of acid catalysts have been evaluated for use in carrying out the dehydration of carbohydrates or of alcohols based on such carbohydrates in order to provide the corresponding dehydration products, such as the above-mentioned HMF, levulinic acid, furfural and isosorbide. Inorganic acids such as H2SO4, H3PO4, and HCl are readily obtained, inexpensive materials but are difficult to regenerate. In order to avoid the regeneration and attendant disposal problems, solid resin catalysts have been tried. Unfortunately, in the presence of water and at the temperatures required for carrying out the dehydration, very few solid acids can demonstrate the activity and stability needed to begin to contemplate a commercially viable process.
WO 20091012445 by Dignan et al. is an example of a proposed process for making HMF using the inorganic acids. In Dignan, HMF is proposed to be made by mixing or agitating an aqueous solution of fructose and inorganic acid catalyst with a water immiscible organic solvent to form an emulsion of the aqueous and organic phases, then heating the emulsion in a flow-through reactor at elevated pressures and allowing the aqueous and organic phases to phase separate. HMF is present in the aqueous and organic phases in about equal amounts, and is removed from both, for example, by vacuum evaporation and vacuum distillation from the organic phase and by passing the aqueous phase through an ion-exchange resin. Residual fructose stays with the aqueous phase. High fructose levels are advocated for the initial aqueous phase, to use relatively smaller amounts of solvent in relation to the amount of fructose reacted.
WO 2009/076627 by Sanborn et al. is an example of a proposed process utilizing solid acid resins. In Sanborn '627, substantially pure HMF, HMF esters or HMF ethers are said to be provided from a carbohydrate source by contacting the carbohydrate source with a solid phase catalyst; “substantially pure” was defined as referencing a purity of HMF of about 70% or greater, optionally about 80% or greater, or about 90% or greater. In one embodiment, a carbohydrate starting material is heated with a solvent in a column, and the heated carbohydrate and solvent are continuously flowed through a solid phase catalyst in the presence of an alcohol to form a HMF ether. The solvent is removed by rotary evaporation to provide a substantially pure HMF ether. In another embodiment, a carbohydrate is heated with an organic acid and a solid catalyst in a solution to form an HMF ester. The resulting HMF ester may then be purified by filtration, evaporation, extraction, and distillation or any combination thereof.
U.S. Pat. Nos. 6,849,748; 7,420,067; 7,439,352; 7,772,412 and 7,982,059 provide examples of prior art methods for producing isohexides (also referred to as anhydrosugar alcohols, anhydrohexitols, anhydroalditols etc) such as isosorbide, from sorbitol from dextrose. Commonly-assigned U.S. Pat. No. 6,849,748 to Moore et al., for example, describes a solvent-free process wherein a sugar alcohol—such as sorbitol—is heated with stirring until molten, and then dehydrated in the presence of a soluble acid or acidic ion exchange resin with stirring, under vacuum (to remove the water product and drive the reaction toward the products) and at all elevated temperature, then the resulting anhydrosugar alcohol is purified by distillation, followed by melt crystallization and/or redistillation. The final, purified product is isolated by centrifugation or filtration. Enumerated preferred acid catalysts include sulfuric acid, phosphoric acid, p-toluenesulfonic acid, and p-methanesulfonic acid. Commonly-assigned U.S. Pat. No. 7,420,067 mentions these same acids, as well as acidic ion exchange resins and acidic zeolite powders as additional options. Successive film evaporators, especially wiped film evaporators under vacuum, are described for use in purifying the product isosorbide.
More recently, U.S. Pat. No. 7,772,412 to Holladay et al. describes a process for making isosorbide wherein sorbitol is fed to a reactor containing a dehydration catalyst and a hydrogenation co-catalyst, with hydrogen being supplied countercurrently to the reactor for removing water as it is formed and for “reducing or eliminating . . . oligomeric or polymeric material in the dehydration product”, to which undesirable color formation had been attributed. Suitable dehydration catalysts include the mineral acid catalysts, solid acid catalysts such as the heteropolyacids, mesoporous silicas, acid clays, sulfated zirconia, molecular sieve materials, cation exchange resins and zeolites, and combinations of any of these. The hydrogenation catalyst is described as typically being a supported metal or multi-metal catalyst. Palladium in particular is described as especially preferable for the metal, with platinum, nickel, cobalt, ruthenium, rhenium, rhodium, iridium and iron also being listed.
Still more recently, U.S. Pat. No. 7,982,059 describes a process for converting aqueous sorbitol to xylitol and isosorbide in the presence of an acid catalyst and without a hydrogenation co-catalyst, more particularly involving reacting an aqueous sorbitol solution with an acid zeolite at about 250 degrees Celsius and a pressure maintained at from about 68 bars to about 80 bars to produce the xylitol and isosorbide.